1. Field of the Invention
This invention relates to an optical pickup which is suitable for use in an information playback apparatus for optically reproducing information, and more particularly to an optical pickup which employs a semiconductor laser as a light source.
2. Description of the Prior Art
In an information playback apparatus which optically reproduces information from an optical video disc or an optical pulse-code-modulated audio disc, an optical pickup employing a semiconductor laser as a light source is used. As the optical pickups, there have heretofore been two types.
The first type is such that light reflected from the information storage medium is fed back to the semiconductor laser being the light source, whereupon a fluctuation in the output power of the semiconductor laser is detected by the photodetector. The optical pickup is the so-called SCOOP (Self-Coupled Optical Pickup).
FIG. 1 is a schematic constructional view of an information playback apparatus employing the SCOOP. Numeral 1 designates a semiconductor laser, and numeral 2 a disc having an information storage medium. By way of example, the medium is disposed on a circular substrate and records information in the form of pits or on the basis of reflection modulation. Numeral 3 indicates a photodetector, numeral 41 a lens for turning light from the semiconductor laser into a collimated beam, and numeral 42 a lens for condensing the laser beam into a spot diameter approximately equal to the wavelength of the light on the surface of the disc 2. This apparatus is characterized in that the disc 2 forms a part of an oscillator for the semiconductor laser. That is, the laser oscillation is caused by an optical resonator which is made up of three mirrors consisting of two facets of a semiconductor chip and the disc 2.
In this apparatus, the light output of the laser oscillation varies depending upon the amount of light fed back by reflection from the disc 2, and hence, information signals recorded on the disc are read out by detecting the variation. The feature of the apparatus is that a small number of constituent components suffice, which makes it possible to render the size small, the weight light and the cost low and which facilitates the adjustments of the optical system. On the other hand, as one of the disadvantages of this system, it is mentioned that the noise level of the signal is high.
The second type optical pickup is such that the reflected light from the optical disc is received by the photodetector directly without being fed back to the semiconductor laser. By way of example, the construction of this optical pickup will be described in connection with the apparatus shown in FIG. 1. A quarter-wave plate and a polarization prism are inserted between the semiconductor laser 1 and the disc 2 in order to prevent the reflected light from the optical disc 2 from being fed back to the semiconductor laser 1. The optical pickup in this case shall hereinbelow be termed the "conventional type optical pickup". It is not the situation in the conventional type optical pickup, either, that the reflected light from the disc is not fed back to the laser at all. In the pulse-code-modulated audio disc and the video disc, replicated discs are mass-produced by the use of synthetic resin. The replicated disc has a slight birefringence on account of a stress which arises during the replication. Accordingly, in case where information are reproduced from the replicated disc with the conventional type optical pickup, part of the reflected light from the disc is fed back to the semiconductor laser due to the birefringence of the replicated disc. Further, the reflected light from the disc is fed back to the semiconductor laser by such causes as slight misadjustment of the optical system including the quarter-wave plate and the polarization prism and deviations in the performance of the components. After all, also in case of the conventional type optical pickup, several % of the reflected light from the disc is fed back to the semiconductor laser and raises the noise level of the laser.
To sum up, the optical pickups employing the semiconductor lasers have the problem of the generation of noise of the semiconductor lasers ascribable to the feedback of the reflected light, whether they are the SCOOP or the conventional type optical pickup. The high noise level of the semiconductor laser incurs degradation in the sound quality in a pulse-code-modulated audio player and degradation in the picture quality in a video disc player.